Tricky problem. front washer pump on golf not working but rear working fine. any suggestions.?

hi i have a mk4 golf tdi and the front windscreen washers dont work but the rear does i have checked the pump on the battery and it works fine works both ways when you swap polarity i have also put a multi meter on the pumps plug and operated the switch when operating the back i get 11.78 V and when operating the front i get -11.38 V i dont understand this should work shouldnt it any suggestions would be great.